MOLLUSCA CONCHIFERA. 231 



boring has been only half completed. This process must 

 be effected in a short time, since both the external openings 

 are of the same size. As the animal becomes further 

 developed, and the tubes enlarged, the partition is broken 

 through. The author believes, from comparison of the 

 size of the borings, that it must be assumed that the animal 

 changes them three times before it reaches its full growth. 

 The interior of the excavation, moreover, is perfectly cir- 

 cular, so that the animal probably, during its growth, 

 turns itself round and round like the Pholus. No such 

 turning on the axis occurs with Modiola lithophagu, in 

 which case the excavation is oval. 



END OF REPORTS FOR 1843. 



